Network Working Group                                       IJ. Wijnands
Internet-Draft                                                 S. Venaas
Intended status: Experimental                        Cisco Systems, Inc.
Expires: May 4, September 11, 2017                                      M. Brig
                                                Aegis BMD Program Office
                                                             A. Jonasson
                           Swedish Defence Material Administration (FMV)
                                                        October 31, 2016
                                                          March 10, 2017

              PIM flooding mechanism and source discovery


   PIM Sparse-Mode uses a Rendezvous Point and shared trees to forward
   multicast packets from new sources.  Once last hop routers receive
   packets from a new source, they may join the Shortest Path Tree for
   the source for optimal forwarding.  This draft defines a new protcol
   mechanism that provides a way to support PIM Sparse Mode (SM) without
   the need for PIM registers, RPs or shared trees.  Multicast source
   information is flooded throughout the multicast domain using a new
   generic PIM flooding mechanism.  This allows last hop routers to
   learn about new sources without receiving initial data packets.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 4, September 11, 2017.

Copyright Notice

   Copyright (c) 2016 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions used in this document . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Testing and deployment experiences  . . . . . . . . . . . . .   3
   3.  A generic PIM flooding mechanism  . . . . . . . . . . . . . .   4
     3.1.  PFM message format  . . . . . . . . . . . . . . . . . . .   4   5
     3.2.  Processing PFM messages . . . . . . . . . . . . . . . . .   5   6
       3.2.1.  Initial checks  . . . . . . . . . . . . . . . . . . .   5   6
       3.2.2.  Processing and forwarding of PFM messages . . . . . .   6
   4.  Distributing Source to Group Mappings . . . . . . . . . . . .   6   7
     4.1.  Group Source Holdtime TLV . . . . . . . . . . . . . . . .   6   7
     4.2.  Originating PFM messages  . . . . . . . . . . . . . . . .   7   8
     4.3.  Processing GSH TLVs . . . . . . . . . . . . . . . . . . .   8
     4.4.  The first packets and bursty sources  . . . . . . . . . .   8   9
     4.5.  Resiliency to network partitioning  . . . . . . . . . . .   9  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   PIM Sparse-Mode uses a Rendezvous Point (RP) and shared trees to
   forward multicast packets to Last Hop Routers (LHR).  After the first
   packet is received by a LHR, the source of the multicast stream is
   learned and the Shortest Path Tree (SPT) can be joined.  This draft
   defines a new mechanism that provides a way to support PIM Sparse
   Mode (SM) without the need for PIM registers, RPs or shared trees.
   Multicast source information is flooded throughout the multicast
   domain using a new generic PIM flooding mechanism.  This mechanism is
   defined in this document, and is modeled after the Bootstrap Router
   mechanism [RFC5059].  By removing the
   need for RPs and shared trees, the PIM-SM procedures are simplified,
   improving router operations, management and making the protocol more
   robust.  Also the data packets are only sent on the SPTs, providing
   optimal forwarding.

1.1.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2.  Terminology

   RP:  Rendezvous Point. Point

   BSR:  Bootstrap Router. Router

   RPF:  Reverse Path Forwarding. Forwarding

   SPT:  Shortest Path Tree. Tree

   FHR:  First Hop Router, directly connected to the source. source

   LHR:  Last Hop Router, directly connected to the receiver. receiver

   PFM:  PIM Flooding Mechanism. Mechanism

   PFM-SA:  PFM Source Announcement. Announcement

   SG Mapping:  Multicast source to group mapping. mapping

2.  Testing and deployment experiences

   A prototype of this specification has been implemented and there has
   been some limited testing in the field.  The prototype was tested in
   a network with low bandwidth radio links.  In this  The network with has frequent
   topology changes and changes, including frequest link or router failures, failures.
   Previously existing mechanisms like PIM-SM with and PIM-DM were tested.

   With PIM-SM the existing RP election is mechanisms were found to be too
   slow.  With PIM-DM, issues were observed with new multicast sources
   starving low bandwidth links even when there are no receivers, in
   some cases such that there was no bandwidth left for prune message.

   For the PFM-SA prototype tests, all routers were configured to send
   PFM-SA for directly connected source and to cache received
   announcements.  Applications such as SIP with multicast subscriber
   discovery, multicast voice conferencing, position tracking and NTP
   were successfully tested.  The tests went quite well.  Packets were
   rerouted as needed and there were no unnecessary forwarding of
   packets.  Ease of configuration was seen as a plus.

3.  A generic PIM flooding mechanism

   The Bootstrap Router mechanism (BSR) [RFC5059] is a commonly used
   mechanism for distributing dynamic Group to RP mappings in PIM.  It
   is responsible for flooding information about such mappings
   throughout a PIM domain, so that all routers in the domain can have
   the same information.  BSR as defined, is only able to distribute
   Group to RP mappings.  We are defining a more generic mechanism that
   can flood any kind of information throughout a PIM domain.  It is not
   necessarily a domain though, it depends on the administrative
   boundaries being configured.  The forwarding rules are identical to
   BSR, except that there is no BSR election and that one can control whether routers should forward
   unsupported data types.  For some types of information it is quite
   useful that it can be distributed without all routers having to
   support the particular type, while there may also be types where it
   is necessary for every single router to support it.  The mechanism
   includes an originator address which is used for RPF checking to
   restrict the flooding, and prevent loops, just like BSR.  Just like BSR it is also sent  Like BSR,
   messages are forwarded hop by hop.  Note that there is no built in equivalent
   to the BSR election mechanism as in BSR, so mechanism;, there can be multiple originators.
   We call this mechanism the PIM Flooding Mechanism (PFM).

3.1.  PFM message format

        0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |PIM Ver| Type  |N|  Reserved   |           Checksum            |
      |            Originator Address (Encoded-Unicast format)        |
      |          Type 1               |          Length 1             |
      |                            Value 1                            |
      |                               .                               |
      |                               .                               |
      |                               .                               |
      |                               .                               |
      |          Type n               |          Length n             |
      |                            Value n                            |
      |                               .                               |
      |                               .                               |

   PIM Version:   Reserved, Checksum Described in [RFC7761].

   Type:   PIM Message Type.  Value (pending IANA) for a PFM message.

   [N]o-Forward bit:   When set, this bit means that the PFM message is
      not to be forwarded.

   Originator Address:   The address of the router that originated the
      message.  This can be any address assigned to the originating
      router, but MUST be routable in the domain to allow successful
      forwarding.  The format for this address is given in the Encoded-
      Unicast address in [RFC7761].

   Type 1..n:   A message contains one or more TLVs, in this case n
      TLVs.  The Type specifies what kind of information is in the

   Length 1..n:   The length of the the value field.

   Value 1..n:   The value associated with the type and of the specified

3.2.  Processing PFM messages

   A router that receives a PFM message MUST perform the initial checks
   specified here.  If the checks fail, the message MUST be dropped.  An
   error MAY be logged, but otherwise the message MUST be dropped
   silently.  If the checks pass, the contents is processed according to
   the processing rules of the included TLVs.

3.2.1.  Initial checks

   In order to do further processing, a message MUST meet the following
   requirements.  The message MUST be from a directly connected neighbor
   for which we have active Hello state, and it MUST have been sent to
   the ALL-PIM-ROUTERS group.  Also, the interface MUST NOT be an
   administrative boundary for PFM.  If No-Forward is not set, it MUST
   have been sent by the RPF neighbor for the originator address.  If
   No-Forward is set, we MUST have restarted within 60 seconds.  In
   pseudo-code the algorithm is as follows:

               if ((DirectlyConnected(PFM.src_ip_address) == FALSE) OR
                   (we have no Hello state for PFM.src_ip_address) OR
                   (PFM.dst_ip_address != ALL-PIM-ROUTERS) OR
                   (Incoming interface is admin boundary for PFM)) {
                    drop the message silently, optionally log error.
               if (PFM.no_forward_bit == 0) {
                   if (PFM.src_ip_address !=
                       RPF_neighbor(PFM.originator_ip_address)) {
                       drop the message silently, optionally log error.
               } else if (more than 60 seconds elapsed since startup)) {
                   drop the message silently, optionally log error.

   Note that src_ip_address is the source address in the IP header of
   the PFM message.  Originator is the originator field inside the PFM
   message, and is the router that originated the message.  When the
   message is forwarded hop by hop, the originator address never
   changes, while the source address will be an address belonging to the
   router that last forwarded the message.

3.2.2.  Processing and forwarding of PFM messages

   When the message is received, the initial checks above must be
   performed.  If it passes the checks, we then for each included TLV
   perform processing according to the specification for that TLV.

   After processing, we forward the message.  Unless otherwise specified
   by the type specification, the TLVs in the forwarded message are
   identical to the TLVs in the received message.  However, if the most
   significant bit in the type field is set (the type value is larger
   than 32767) and we do not support the type, then that particular type
   should be omitted from the forwarded messages.  The message is
   forwarded out of all interfaces with PIM neighbors (including the
   interface it was received on).

4.  Distributing Source to Group Mappings

   The generic flooding mechanism (PFM) defined in the previous section
   can be used for distributing source to group mappings about active
   multicast sources throughout a PIM domain.  A Group Source Holtime
   (GSH) TLV is defined for this purpose.

4.1.  Group Source Holdtime TLV

        0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |          Type = 0               |          Length             |
      |              Group Address (Encoded-Group format)             |
      |            Src Count          |        Src Holdtime           |
      |            Src Address 1 (Encoded-Unicast format)             |
      |            Src Address 2 (Encoded-Unicast format)             |
      |                               .                               |
      |                               .                               |
      |            Src Address m (Encoded-Unicast format)             |

   Type:   This TLV has type 0.

   Length:   The length of the value.

   Group Address:   The group we are announcing sources for.  The format
      for this address is given in the Encoded-Group format in

   Src Count:   How many unicast encoded sources address encodings

   Src Holdtime:   The Holdtime (in seconds) for the corresponding

   Src Address:   The source address for the corresponding group.  The
      format for these addresses is given in the Encoded-Unicast address
      in [RFC7761].

4.2.  Originating PFM messages

   A PFM message MAY contain one or more Group Source Holdtime (GSH)
   TLVs.  This is used to flood information about active multicast
   sources.  Each FHR that is directly connected to an active multicast
   source originates PFM messages containing GSH TLVs.  How a multicast
   router discovers the source of the multicast packet and when it
   considers itself the FHR follows the same procedures as the
   registering process described in [RFC7761].  When a FHR has decided
   that a register needs to be sent per [RFC7761], the SG is not
   registered via the PIM SM register procedures, but the SG mapping is
   included in an GSH TLV in a PFM message.  Note, only the SG mapping
   is distributed in the message, not the entire packet as would have
   been done with a PIM register.  The router originating the PFM
   messages includes one of its own addresses in the originator field.
   Note that this address SHOULD be routeable due to RPF checking.  The
   PFM messages containing the GSH TLV are periodically sent for as long
   as the multicast source is active, similar to how PIM registers are
   periodically sent.  The default announcement period is 60 seconds,
   which means that as long as the source is active, it is included in a
   PFM message originated every 60 seconds.  The holdtime for the source
   is by default 210 seconds.  Other values MAY be configured, but the
   holdtime MUST be either zero, or larger than the announcement period.
   It is RECOMMENDED to be 3.5 times the announcement period.  A source
   MAY be announced with a holdtime of zero to indicate that the source
   is no longer active.

   If an implementation supports originating GSH TLVs with different
   holdtimes for different sources, it can if needed send multiple TLVs
   with the same group address.  Due to the format, all the sources in
   the same TLV have the same holdtime.

4.3.  Processing GSH TLVs

   A router that receives a PFM message containing GSH TLVs SHOULD parse
   the message and store each of the GSH TLVs as SG mappings with a
   holdtimer started with the advertised holdtime.  For each group that
   has directly connected receivers, this router SHOULD send PIM (S,G)
   joins for all the SG mappings advertised in the message for the
   group.  The SG mappings are kept alive for as long as the holdtimer
   for the source is running.  Once the holdtimer expires a PIM router
   MAY send a PIM (S,G) prune to remove itself from the tree.  However,
   when this happens, there should be no more packets sent by the
   source, so it may be desirable to allow the state to time out rather
   than sending a prune.

   Note that a holdtime of zero has a special meaning.  It is to be
   treated as if the source just expired, and state to be removed.
   Source information MUST NOT be removed due to the source being
   omitted in a message.  For instance, if there is a large number of
   sources for a group, there may be multiple PFM messages, each message
   containing a different list of sources for the group.

4.4.  The first packets and bursty sources

   The PIM register procedure is designed to deliver Multicast packets
   to the RP in the absence of a Shortest Path Tree (SPT) from the RP to
   the source.  The register packets received on the RP are decapsulated
   and forwarded down the shared tree to the LHRs.  As soon as an SPT is
   built, multicast packets would flow natively over the SPT to the RP
   or LHR and the register process would stop.  The PIM register process
   ensures packet delivery until an SPT is in place reaching the FHR.
   If the packets were not unicast encapsulated to the RP they would be
   dropped by the FHR until the SPT is setup.  This functionality is
   important for applications where the initial packet(s) must be
   received for the application to work correctly.  Another reason would
   be for bursty sources.  If the application sends out a multicast
   packet every 4 minutes (or longer), the SPT is torn down (typically
   after 3:30 minutes of inactivity) before the next packet is forwarded
   down the tree.  This will cause no multicast packet to ever be
   forwarded.  A well behaved application should be able to deal with
   packet loss since IP is a best effort based packet delivery system.
   But in reality this is not always the case.

   With the procedures defined in this document the packet(s) received
   by the FHR will be dropped until the LHR has learned about the source
   and the SPT is built.  That means for bursty sources or applications
   sensitive for the delivery of the first packet this solution would
   not be very applicable.  This solution is mostly useful for
   applications that don't have strong dependency on the initial
   packet(s) and have a fairly constant data rate, like video
   distribution for example.  For applications with strong dependency on
   the initial packet(s) we recommend using PIM Bidir [RFC5015] or SSM
   [RFC4607].  The protocol operations are much simpler compared to PIM
   SM, it will cause less churn in the network and both guarantee best
   effort delivery for the initial packet(s).

   Another solution to address the problems described above is
   documented in [I-D.ietf-magma-msnip].  This proposal allows for a
   host to tell the FHR its willingness to act as Source for a certain
   Group before sending the data packets.  LHRs have time to join the
   SPT before the host starts sending which would avoid packet loss.
   The SG mappings announced by [I-D.ietf-magma-msnip] can be advertised
   directly in SG messages, allowing a nice integration of both
   proposals.  The life time of the SPT is not driven by the liveliness
   of Multicast data packets (which is the case with PIM SM), but by the
   announcements driven via [I-D.ietf-magma-msnip].  This will also
   prevent packet loss due to bursty sources.

4.5.  Resiliency to network partitioning

   In a PIM SM deployment where the network becomes partitioned, due to
   link or node failure, it is possible that the RP becomes unreachable
   to a certain part of the network.  New sources that become active in
   that partition will not be able to register to the RP and receivers
   within that partition are not able to receive the traffic.  Ideally
   you would want to have a candidate RP in each partition, but you
   never know in advance which routers will form a partitioned network.
   In order to be fully resilient, each router in the network may end up
   being a candidate RP.  This would increase the operational complexity
   of the network.

   The solution described in this document does not suffer from that
   problem.  If a network becomes partitioned and new sources become
   active, the receivers in that partitioned will receive the SG
   Mappings and join the source tree.  Each partition works
   independently of the other partition(s) and will continue to have
   access to sources within that partition.  As soon as the network
   heals, the SG Mappings are re-flooded into the other partition(s) and
   other receivers can join to the newly learned sources.

5.  Security Considerations

   The security considerations are mainly similar to what is documented
   in [RFC5059].  It is a concern that rogue devices can inject packets
   that are flooded throughout a domain.  PFM packets must only be
   accepted from a PIM neighbor.  Deployments may use mechanisms for
   authenticating PIM neighbors.  For PFM-SA it is an issue that
   injected packets from a rogue device could send SG mappings for a
   large number of source addresses, causing routers to use memory
   storing these mappings, and also if they have interest in the groups,
   build Shortest Path Trees for sources that are not actually active.

6.  IANA considerations

   This document requires the assignment of a new PIM message type for
   the PIM Flooding Mechanism (PFM).  IANA is also requested to create a
   registry for PFM TLVs, with type 0 assigned to the "Source Group
   Holdtime" TLV.  Values in the range 1-65535 are "Unassigned".
   Assignments for the registry are to be made according to the policy
   "IETF Review" as defined in [RFC5226].

7.  Acknowledgments

   The authors would like to thank Arjen Boers for contributing to the
   initial idea, and Yiqun Cai and Dino Farinacci for their comments on
   the draft.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5059]  Bhaskar, N., Gall, A., Lingard, J., and S. Venaas,
              "Bootstrap Router (BSR) Mechanism for Protocol Independent
              Multicast (PIM)", RFC 5059, DOI 10.17487/RFC5059, January
              2008, <>.

   [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
              Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
              Multicast - Sparse Mode (PIM-SM): Protocol Specification
              (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
              2016, <>.

8.2.  Informative References

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,

   [RFC5015]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
              "Bidirectional Protocol Independent Multicast (BIDIR-
              PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,

              Fenner, B., Haberman, B., Holbrook, H., Kouvelas, I., and
              S. Venaas, "Multicast Source Notification of Interest
              Protocol (MSNIP)", draft-ietf-magma-msnip-06 (work in
              progress), March 2011.

Authors' Addresses

   IJsbrand Wijnands
   Cisco Systems, Inc.
   De kleetlaan 6a
   Diegem  1831

   Stig Venaas
   Cisco Systems, Inc.
   Tasman Drive
   San Jose  CA  95134


   Michael Brig
   Aegis BMD Program Office
   17211 Avenue D, Suite 160
   Dahlgren  VA 22448-5148


   Anders Jonasson
   Swedish Defence Material Administration (FMV)
   Loennvaegen 4
   Vaexjoe  35243